56 ANXALS NEW YORE ACADEMY OF SCIENCES 



In the norite family, however, this structure is commonest and best 

 developed. As remarked above, it seems to be a general rule that the 

 finer grained a norite is, the simpler it is, i. e., a very fine-grained norite 

 is composed chiefly of feldspar with considerable hypersthene, while the 

 coarser varieties carry in addition either hornblende, or biotite and augite. 

 The fine-grained simple norite is never found in large areas, but always 

 as inclusions in the coarser, and therefore more complex varieties. It 

 often occurs in biotite norite, for example, as small, rounded flow-like 

 patches, or again as streaks : or it may be banded with the coarser rock. 

 In this case, the chemical difference is not so great, as the accompanying 

 analyses show. 



Xow, if the simpler norite be not quite so fine grained, it will not be 

 entirely pure ; and this is the case in most places. In Plate IV, fig. 1, the 

 mass of the rock is a biotite augite norite, while the white streak is 

 merely a finer and therefore simpler facies, containing only small amounts 

 of biotite and augite. In one place, it shows an included patch of the 

 coarser rock. The ledge shown is about 20 feet high. Plate IV, fig. 2, 

 shows the same thing in better development, so that the rock might easily 

 be mistaken for a real metamorphic gneiss. A number of other equally 

 good instances might be shown, for the structure is quite common; but 

 these suffice to show its general aspect. 43 



From what has been said above, it is evident that this structure cannot 

 be due to ordinarv shearing, and we are therefore obliged to search for 

 another explanation. 



Eliminating Dana's idea of worked-over volcanic ashes, and "Williams's 

 suggestion of the ordinary regional metamorphism of igneous rocks, we 

 are thrown back on some force concomitant in its action with the cooling 

 of the rock. Since the several layers or streaks are always quite different 

 in mineralogical composition at least and more or less so in chemical, it 

 is evidently a question of magmatic differentiation. It is inconceivable 

 that the structure be due to the differentiation of a magma in situ — after 

 it had reached its present position — since the differentiation is into bands 

 which bear no definite relation to the borders of the magma ; and the 

 idea of successive intrusions — first of a light band and then of a dark — is 

 equally inapplicable, since even when there is a sharp line of demarcation 



43 The classic locality for this structure Is on the Isle of Skye, in Tertiary gabbro, 

 where it was first described as such by Giekie and Teall. (Quart. Jour. Geol. Soc, I, 

 646. 1894.) It has also been described in wonderful development by A. G. H5gbom, 

 from the Island of OrnO, just south of Stockholm. ("Zur Petrographie von Orno Huf- 

 vud," Bull, of the Geol. Instit. of Upsala, X, 150.) F. D. Adams has noted a striking 

 development of the structure near Montreal, which he will describe in a forthcoming 

 paper. 



